Orbital Diagrams Chemistry Tutorial

Key Concepts

• An orbital diagram, or orbital box diagram, is a way of representing the electron configuration of an atom.
• A box, line, or circle, is drawn to represent each orbital in the electron configuration
  (using the Aufau Principle to order the orbitals and hence the boxes, lines or circles, as shown below)

  1s

  →

  2s

  →

  2px

  2py

  2pz

  →

  3s

  →

  3px

  3py

  3pz

  →

  4s

  →

  3dxy

  3dxz

  3dyz

  3dx2-y2

  3dz2

  →

  4px

  4py

  4pz

  →

  5s

• Arrows (or half arrows) are used to represent the electrons occupying the orbitals.
• Arrows (or half arrows) can point up or down:

  ⚛ electrons with "up spin" are drawn either as upward pointing arrows, ↑, or half arrows, ↿

  ⚛ electrons with "down spin" are drawn either as downward pointing arrows, ↓, or half arrows, ⇂

• A maximum of 2 arrows can be drawn in each box
  (because a maximum of 2 electrons can occupy an orbital)
• When 2 arrows occupy the same box we refer to the electrons as paired, and we must apply the Pauli Exclusion Principle so that these arrows face in opposite directions (one "spin up", one "spin down").
  (The paired electrons are said to have antiparallel spin).

  ↑↓

  or

  ↿⇂

• One arrow is positioned in each box according to Hund's Rule which tells us to maximise the number of unpaired electrons in orbitals of the same subshell, and, to give those electrons the same "spin" (parallel spin).

  s subshell		s subshell		p subshell
  ↑↓		↑↓		↑	↑
  1s		2s		2px	2py	2pz

• For atoms of "d block" elements, in Period 4 for example, the crowding of the 4s and 3d orbitals results in the application of Hund's Rule to all these orbitals.
  Note the orbital diagrams for chromium and copper shown below:

  Cr	↑↓		↑↓		↑↓	↑↓	↑↓		↑↓		↑↓	↑↓	↑↓		↑	↑	↑	↑	↑		↑
  	1s		2s		2px	2py	2pz		3s		3px	3py	3pz		3dxy	3dxz	3dyz	3dx2−y2	3dz2		4s

  Cu	↑↓		↑↓		↑↓	↑↓	↑↓		↑↓		↑↓	↑↓	↑↓		↑↓	↑↓	↑↓	↑↓	↑↓		↑
  	1s		2s		2px	2py	2pz		3s		3px	3py	3pz		3dxy	3dxz	3dyz	3dx2−y2	3dz2		4s

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Understanding and Drawing Orbital Diagrams

You should already be familiar with the Bohr Model of the Atom which states that electrons exist in discrete energy levels (or electron shells).

The principal quantum number, n, tells us the energy level (or electron shell) that the electron is found in.

• n=1 is the first energy level and is designated the K shell
• n=2 is the second energy level and is designated the L shell
• n=3 is the third energy level and is designated the M shell
• n=4 the fourth energy level and is designated the N shell

We can use this to write the simple electron configuration of an atom just by noting how many electrons are occupying each energy level (or shell).

Nitrogen (Z = 7): 2,5

2 electrons in the 1^st energy level (n=1, K shell)

5 electrons in the second energy level (n=2, L shell)

You should also be aware that each energy level (or shell) is further divided into energy sub-levels (or sub-shells).

• n=1, 1^st energy level (K shell) has only one sublevel (s)
• n=2, 2^nd energy level (L shell) has 2 sublevels (s and p)
• n=3, 3^rd energy level (M shell) has 3 sublevels (s, p and d)
• n=4, 4^th energy level (N shell) has 4 sublevels (s, p, d and f)

Using this information, and the Aufbau Principle, we can write an electron configuration using subshells or sublevels, filling each sublevel in the following order

Nitrogen (Z = 7): 1s² 2s² 2p³

2 electrons in the s sublevel of the 1^st energy level (n=1, K shell)

2 electrons in the s sublevel of the 2^nd energy level (n=2, L shell)

3 electrons in the p sublevel of the second energy level (n=2, L shell)

Each sublevel (subshell) is divided up into a set of orbitals.
The number of orbitals is given by n²

• 1^st energy level (K shell) has 1² = 1 orbital (1 × s orbital)
• 2^nd energy level (L shell) has 2² = 4 orbitals (1 × s orbital and 3 × p orbitals)
• 3^rd energy level (M shell) has 3² = 9 orbitals (1 × s orbital, 3 × p orbitals and 5 × d orbitals)
• 4^th energy level (N shell) has 4² = 16 orbitals (1 × s orbital, 3 × p orbitals, 5 × d orbitals and 7 × f orbitals)

Furthermore, each orbital has a particular shape.
Another quantum number, the azimuthal quantum number (l) tells us the shape of the orbital that the electron is occupying.

• an s orbital is spherical
• each of the three p orbitals are made up of 2 lobes with a node separating them
• 4 of the 5 d orbitals are made up of 2 lobes with a node separating them, while the fifth d orbital has a donut separating the two lobes

Another quantum number, the magnetic quantum number, m_l, tells us the orientation of the orbital in space:

• an s orbital is spherical so it has only one orientation in space
• the lobes of a p orbital can be oriented along the x, y or z axes, so each of these is designated respectively as a p_x, p_y, and p_z orbital.
• similarly the lobes of the d orbitals are each orientated differently in space, these are designated as the d_xy, d_xz, d_yz, d_x²−y², and d_z²

All of this information together allows us to draw a set of boxes to represent the orbitals of each energy level (shell).
We can build up a set of boxes to represent the energy level (shells) and orbitals for the atoms of each element in each period of the periodic table:

Period 1, we just need one box to represent the 1s orbital:

Period 2, the 1s orbital of 1^st energy level (K shell), as well as an s orbital and 3 p orbitals in the 2^nd energy level (L shell) :

Period 3, the 1s orbital of 1^st energy level (K shell), as well as an s orbital and 3 p orbitals in the 2^nd energy level (L shell) and the s and p orbitals of the 3^rd energy level (M shell):

Period 4, the 3d orbitals become available to the "d-block elements" or transition metals straight after the "s-block" elements.
We will keep orbitals of the same energy level together, but remember that electrons will occupy the 4s orbital before the 3d orbitals (in accordance with the Aufbau Principle):

In order to build up an orbital diagram for an atom of each element, we will need to place electrons into the boxes we have drawn.

The first thing we need to remember is that an orbital can hold a maximum of 2 electrons: that is, an orbital can hold 0 electrons, 1 electron or 2 electrons.
But, the Pauli Exclusion Principle tells us that no two atoms in a given atom can be exactly the same, if we already have an electron in the 1s orbital, then there must be something different about the second electron we place in the same orbital.
This difference is called its spin quantum number, m_s, and it has values of either +½ or −½.
In orbital diagrams we represent these two different states for each electron as two different arrows:

• "spin up" arrows, ↑, or "spin up" half arrows, ↿
• "spin down" arrows, ↓, or "spin down" half arrows, ⇂

If 2 arrows (or half arrows) are paired (occupy the same box) then they must face in opposite directions (they are said to have antiparallel spin).

The following diagram illustrates correct pairs of electrons in an orbital box, and incorrect pairs of electrons in an orbital box:

But before we start pairing electrons up in orbitals, we need to position electrons in accordance with Hund's Rule: electrons in a given subshell tend to remain unpaired with parallel spins. This means that if we have 3 electrons to place into a p-subshell (a set of 3 p orbitals), we put 1 arrow in each of the 3 boxes BEFORE we start placing 2 electrons in any of the boxes. Each of these three arrows must be the same, for example, all "up spin" (parallel spin).
The diagram below shows correct and incorrect allocations of 3 electrons to the p orbital boxes of the p subshell:

For example, an atom of nitrogen has 7 electrons (number of electrons = atomic number = Z = 7)

Nitrogen is a period 2 element, and is a p-block element, therefore:
The 1^st energy level is full: 1s²
In the 2^nd energy level the s orbital is full, 2s² and there are 3 electrons in the p orbitals, 2p³

So we draw the required orbital boxes for the orbital diagram using the Aufbau Principle to get the correct order for filling the boxes:

Next we populate the orbitals with electrons (arrows).

Place an arrow (or half arrow) in the first box (1s box) to represent the first electron:

Now we need to place the second arrow to represent the second electron.
We must complete the s subshell before we can begin the next energy level, so the second arrow is placed in the same 1s box.
Because 2 arrows will occupy the same box, we apply the Pauli Exclusion Principle so that these two arrows face the opposite way (antiparallel spin):

Now we place the third arrow (or half arrow) to represent the third electron in the 2s box:

The fourth arrow goes in the same 2s box because we need to complete the s orbital of the second energy level before moving on to the p orbitals of the second energy level:
We apply the Pauli Exclusion Principle so that these arrows face in opposite directions (antiparallel spin)

The fifth arrow must go in a p orbital of the second energy level (say, the 2p_x orbital):

Now the sixth arrow WILL NOT pair up with the previous arrow because there are 2 other orbitals available in this sublevel, so, we use Hund's Rule to place this arrow in the next p orbital (say the 2p_y) box AND both arrows must face the same direction (have parallel spin):

The seventh arrow (the final arrow) will not pair up with either of the previous 2 arrows because there is still an empty p orbital available in the second energy level.
We apply Hund's Rule again so that this arrow is positioned in the last available p orbital (2p_z) and facing the same direction as the other two arrows (parallel spin).
The completed orbital diagram for an atom of nitrogen is therefore given below:

In the following sections we present the orbital diagrams for atoms of the first 36 elements of the Periodic Table.
Before you look at the next section, why don't you try to draw the orbital diagrams for the atoms in each Period first, then compare your orbital diagrams with the orbital diagrams below.
If your orbital diagram is not the same as the one given below, read through the explanation and try again.

Orbital Diagrams for Period 1 Elements

The Aufbau Principle tells us that atoms of period 1 elements are filling the first energy level (K shell) which is composed of just one s orbital.
The maximum number of electrons that can occupy an orbital is 2.
When 2 electrons occupy the same orbital we apply the Pauli Exclusion Principle so that one electron has a spin quantum number (m_s) of +½ (spin up, ↑ or ↿) and the other electron has spin quantum number (m_s) of −½ (spin down, ↓ or ⇂).

Hydrogen : atomic number (Z) = 1.
Aufbau Principle: Just 1 electron occupying the s orbital of the 1^st energy level (K shell)

electron configuration (shells): 1

electron configuration (sub-shells): 1s¹

orbital diagram (orbital box diagram) : just one electron occupying one box

Helium : atomic number (Z) = 2
Aufbau Principle: 2 electrons occupying the same s orbital of the 1^st energy level (K shell)

electron configuration (shells): 2

electron configuration (sub-shells): 1s²

orbital diagram (orbital box diagram) : apply the Pauli Exclusion Principle so that one electron has the opposite spin to the other (one "up" and one "down")

Orbital Diagrams for Period 2 Elements

The Aufbau Principle tells us that the first energy level (K shell) containing the 1s orbital was completed with the last Period 1 element, helium [He].
Each Period 2 element therefore begins building on this completed 1s orbital (1s²).
The orbital diagram for each Period 2 element will begin with a box occupied by 2 arrows (one up, one down) representing the completed 1s orbital (1s²).
Electrons are then added to the second energy level (L shell) which is made up of one s orbital and 3 p orbitals (p_x, p_y, p_z).
"s block" elements are filling the s orbital, "p block" elements have filled the s orbital and are adding electrons to the p orbitals.
We apply Hund's Rule to maximise the number of unpaired electrons in the p orbitals, that is, electrons will occupy the p orbitals singly until there is 1 electron in each p orbital, after that we must start pairing-up the electrons in the p orbitals.
The maximum number of electrons that can occupy an orbital is 2.
When 2 electrons occupy the same orbital we apply the Pauli Exclusion Principle so that one electron has a spin quantum number (m_s) of +½ (spin up, ↑ or ↿) and the other electron has spin quantum number (m_s) of −½ (spin down, ↓ or ⇂).

Lithium : atomic number (Z) = 3 (s block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), the third electron occupies the s orbital of the second energy level (L shell)

electron configuration (shells): 2,1

electron configuration (sub-shells): 1s² 2s¹

condensed electron configuration: [He] 2s¹

orbital diagram (orbital box diagram) : 1s box has 2 arrows (as per helium above), 2s box has 1 arrow

Beryllium : atomic number (Z) = 4 (s block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), the third and fourth electrons occupy the s orbital of the second energy level (L shell)

electron configuration (shells): 2,2

electron configuration (sub-shells): 1s² 2s²

condensed electron configuration: [He] 2s²

orbital diagram (orbital box diagram) : 1s box has 2 arrows (as per helium above), 2s box has 2 arrows so we apply the Pauli Exclusion Principle so that one electron is "spin up" and the other is "spin down"

Boron : atomic number (Z) = 5 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 3 electrons occupy the second energy level (L shell), 2 electrons in the 2s orbital as per boron above, but the 5^th electron occupies the p sub-shell of the second energy level (L shell)

electron configuration (shells): 2,3

electron configuration (sub-shells): 1s² 2s² 2p¹

condensed electron configuration: [He] 2s² 2p¹

orbital diagram (orbital box diagram) : 1s box has 2 arrows (as per helium above), 2s box has 2 arrows as per boron above, but now we see that there are 3 orbitals that make up the p-subshell (p_x, p_y, p_z), and into one of these we place the 5^th arrow

Carbon : atomic number (Z) = 6 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 4 electrons occupy the second energy level (L shell), 2 electrons in the 2s orbital as per boron above, but the 5^th and 6^th electrons occupy the p sub-shell of the second energy level (L shell)

electron configuration (shells): 2,4

electron configuration (sub-shells): 1s² 2s² 2p²

condensed electron configuration: [He] 2s² 2p²

orbital diagram (orbital box diagram) : 1s box has 2 arrows (as per helium above), 2s box has 2 arrows as per boron above, but now we see that there are 3 orbitals that make up the p-subshell (p_x, p_y, p_z), into which we need to place 2 arrows. So, we apply Hund's Rule so that we maximise the number of unpaired electrons in all the 2p orbitals, and, we give those electrons parallel spin (arrows point in the same direction):

Nitrogen : atomic number (Z) = 7 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 5 electrons occupy the second energy level (L shell), 2 of these electrons are in the 2s orbital as per boron above, but the 5^th, 6^th and 7^th electrons occupy the p sub-shell of the second energy level (L shell)

electron configuration (shells): 2,5

electron configuration (sub-shells): 1s² 2s² 2p³

condensed electron configuration: [He] 2s² 2p³

orbital diagram (orbital box diagram) : 1s box has 2 arrows (as per helium above), 2s box has 2 arrows as per boron above, but now we see that there are 3 orbitals that make up the p-subshell (p_x, p_y, p_z), into which we need to place 3 arrows. So, we apply Hund's Rule so that we maximise the number of unpaired electrons in all the 2p orbitals, and, we give those electrons parallel spin (arrows point in the same direction):

Oxygen : atomic number (Z) = 8 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 6 electrons occupy the second energy level (L shell), 2 of these electrons are in the 2s orbital as per boron above, but the 5^th, 6^th, 7^th and 8^th electrons occupy the p sub-shell of the second energy level (L shell)

electron configuration (shells): 2,6

electron configuration (sub-shells): 1s² 2s² 2p⁴

condensed electron configuration: [He] 2s² 2p⁴

orbital diagram (orbital box diagram) : 1s box has 2 arrows (as per helium above), 2s box has 2 arrows as per boron above, but now we see that there are 3 orbitals that make up the p-subshell (p_x, p_y, p_z), into which we need to place 4 arrows.
So, we apply Hund's Rule so that we maximise the number of unpaired electrons in all the 2p orbitals, and unpaired electrons will have parallel spin.
This means there will be a pair of electrons in one of the boxes.
Apply the Pauli Exclusion Principle to the paired electrons so that one electron is "spin up" and the other is "spin down"

Fluorine : atomic number (Z) = 9 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 7 electrons occupy the second energy level (L shell), 2 of these electrons are in the 2s orbital as per boron above, but the 5^th, 6^th, 7^th, 8^th and 9^th electrons occupy the p sub-shell of the second energy level (L shell)

electron configuration (shells): 2,7

electron configuration (sub-shells): 1s² 2s² 2p⁵

condensed electron configuration: [He] 2s² 2p⁵

orbital diagram (orbital box diagram) : 1s box has 2 arrows (as per helium above), 2s box has 2 arrows as per boron above, but now we need to place 5 arrows in the 3 orbitals that make up the p-subshell (p_x, p_y, p_z).
Apply Hund's Rule to maximise the number of unpaired electrons in all the 2p orbitals which means there will be a pair of electrons in two of the p-subshell boxes.
Apply the Pauli Exclusion Principle to the paired electrons in each box so that one electron is "spin up" and the other is "spin down"

Neon : atomic number (Z) = 10 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the second energy level (L shell), 2 of these electrons are in the 2s orbital as per boron above, but the 5^th, 6^th, 7^th, 8^th, 9^th and 10^th electrons occupy the p sub-shell of the second energy level (L shell)

electron configuration (shells): 2,8

electron configuration (sub-shells): 1s² 2s² 2p⁶

condensed electron configuration: [He] 2s² 2p⁶

orbital diagram (orbital box diagram) : 1s box has 2 arrows (as per helium above), 2s box has 2 arrows as per boron above, but now we need to place 6 arrows in the 3 orbitals that make up the p-subshell (p_x, p_y, p_z).
Each p orbital (p_x, p_y, and p_z) will be be occupied by a pair of electrons.
Apply the Pauli Exclusion Principle to the paired electrons in each box so that one electron is "spin up" and the other is "spin down"

Orbital Diagrams for Period 3 Elements

The electronic configuration of atoms of all Period 3 elements begins with a completed 1^st and 2^nd energy level (filled K and L shells), that is, with the electron configuration of the last element of Period 2, the Noble gas neon, [Ne].
The valence electrons (outermost shell electrons, or highest energy level electrons) begin occupying the 3^rd energy level (M shell).
The third energy level (M shell) of Period 3 elements is made up of one s orbital and 3 p orbitals (p_x, p_y, p_z).
"s block" elements are filling the s orbital, p block elements have filled the s orbital and are adding electrons to the p orbitals.
We apply Hund's Rule to maximise the number of unpaired electrons in the p orbitals, that is, electrons will occupy the p orbitals singly until there is 1 electron in each p orbital, after that we must start pairing-up the electrons in the p orbitals.
The maximum number of electrons that can occupy an orbital is 2.
When 2 electrons occupy the same orbital we apply the Pauli Exclusion Principle so that one electron has a spin quantum number (m_s) of +½ (spin up, ↑ or ↿) and the other electron has spin quantum number (m_s) of −½ (spin down, ↓ or ⇂).

Sodium : atomic number (Z) = 11 (s block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 1 electron occupies the third energy level (M shell) in an s orbital.

electron configuration (shells): 2,8,1

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s¹

condensed electron configuration: [Ne] 3s¹

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y and 2p_z boxes, with 1 electron placed in the 3s box

Magnesium : atomic number (Z) = 12 (s block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 2 electrons occupy the third energy level (M shell) in an s orbital.

electron configuration (shells): 2,8,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s²

condensed electron configuration: [Ne] 3s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y and 2p_z boxes, with 2 electrons placed in the 3s box
Apply the Pauli Exclusion Principle so that for paired electrons, one electron has "up spin" and the other has "down spin"

Aluminium : atomic number (Z) = 13 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 3 electrons occupy the third energy level (M shell) 2 of these electrons in an s orbital and the 3^rd electron in one of the 3 available p-orbitals (p_x, p_y, p_z).

electron configuration (shells): 2,8,3

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p¹

condensed electron configuration: [Ne] 3s² 3p¹

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y and 2p_z and 3s boxes, with 1 electron in a 3p box.

Silicon : atomic number (Z) = 14 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 4 electrons occupy the third energy level (M shell) 2 of these electrons in an s orbital while the 3^rd and 4^th electrons occupy the available p-subshell.

electron configuration (shells): 2,8,4

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p²

condensed electron configuration: [Ne] 3s² 3p²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y and 2p_z and 3s boxes, with 2 electrons occupying 3p boxes.
Apply Hund's Rule: maximise the number of unpaired electrons in all the 3p orbitals, that is, there will be 2 unpaired electrons and these will have parallel spin (arrows facing the same direction)

Phosphorus : atomic number (Z) = 15 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 5 electrons occupy the third energy level (M shell) 2 of these electrons in an s orbital while the 3^rd, 4^th and 5^th electrons occupy the available p-subshell.

electron configuration (shells): 2,8,5

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p³

condensed electron configuration: [Ne] 3s² 3p³

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y and 2p_z and 3s boxes, with 3 electrons occupying 3p boxes.
Apply Hund's Rule: maximise the number of unpaired electrons in all the 3p orbitals, that is, there will be 3 unpaired electrons and they will have parallel spin (arrows pointing in the same direction)

Sulfur : atomic number (Z) = 16 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 6 electrons occupy the third energy level (M shell) 2 of these electrons in an s orbital while the 3^rd, 4^th, 5^th and 6^th electrons occupy the available p-subshell.

electron configuration (shells): 2,8,6

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁴

condensed electron configuration: [Ne] 3s² 3p⁴

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y and 2p_z and 3s boxes, with 4 electrons occupying 3p boxes.
Apply Hund's Rule: maximise the number of unpaired electrons in all the 3p orbitals, that is, there will be 2 unpaired electrons with parallel spin, and, 1 pair of electrons.
Apply the Pauli Exclusion Principle to the pairs of electrons: 1 electron is "spin up" and the other is "spin down"

Chlorine : atomic number (Z) = 17 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 7 electrons occupy the third energy level (M shell) 2 of these electrons in an s orbital while the 3^rd, 4^th, 5^th, 6^th and 7^th electrons occupy the available p-subshell.

electron configuration (shells): 2,8,7

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁵

condensed electron configuration: [Ne] 3s² 3p⁵

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y and 2p_z and 3s boxes, with 5 electrons occupying all three of the 3p boxes.
Apply Hund's Rule: maximise the number of unpaired electrons in all the 3p orbitals, that is, there will be 1 unpaired electron and 2 pairs of electrons.
Apply the Pauli Exclusion Principle to the pairs of electrons: 1 electron is "spin up" and the other is "spin down"

Argon : atomic number (Z) = 18 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the third energy level (M shell) 2 of these electrons in an s orbital while the 3^rd, 4^th, 5^th, 6^th, 7^th and 8^th electrons occupy the available p-subshell.

electron configuration (shells): 2,8,8

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶

condensed electron configuration: [Ne] 3s² 3p⁶

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y and 2p_z and 3s boxes, with 6 electrons occupying 3p boxes, that is, each 3p orbital will contain a pair of electrons.
Apply the Pauli Exclusion Principle to the pairs of electrons: 1 electron is "spin up" and the other is "spin down"

Orbital Diagrams for Period 4 Elements

The atoms of Period 4 elements have completed the first energy level (K shell) and the second energy level (L shell).
The 3s and 3p orbitals are also full, so the inner electron shells have the electron configuration of the last period 3 element, the Noble gas Argon [Ar].
The atoms of Period 4 "s block" elements are adding electrons to the s-subshell of the fourth energy level (N shell).
The five d orbitals of the third energy level become available to the "d block" elements (transition metals) in period 4. These d orbitals are designated 3d_xy, 3d_xz, 3d_yz, 3d_x²−y² and 3d_z²
The "p block" elements of Period 4 are filling the p-subshell made up of three orbitals (4p_x, 4p_y and 4p_z) of the fourth energy level (N shell).
We apply Hund's Rule to maximise the number of unpaired electrons and the Pauli Exclusion Principle to allocate one electron of a pair electrons in an orbital a spin quantum number (m_s) of +½ (spin up, ↑ or ↿) and the other electron of the pair is given a spin quantum number (m_s) of −½ (spin down, ↓ or ⇂).
With regards to chromium (Z = 24) we note that the atom gains stability by half-filling both the 4s orbital and all of the 3d orbitals.
In the case of copper (Z = 29) we note that the atom gains stability by having pairs of electrons in all its 3d orbitals but one unpaired electron in the 4s orbital.

Potassium : atomic number (Z) = 19 (s block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the third energy level (M shell) 2 of these electrons occupy the 3s orbital and 6 electrons occupy the available p-subshell. One unpaired electron occupies the s orbital of the fourth energy level (N shell).

electron configuration (shells): 2,8,8,1

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

condensed electron configuration: [Ar] 4s¹

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and 1 electron occupies the 4s orbital.

Calcium : atomic number (Z) = 20 (s block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the third energy level (M shell) 2 of these electrons occupy the 3s orbital and 6 electrons occupy the available p-subshell. One pair of electrons occupy the s orbital of the fourth energy level (N shell).

electron configuration (shells): 2,8,8,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

condensed electron configuration: [Ar] 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and 2 electrons occupy the 4s orbital, so we apply the Pauli Exclusion Principle by making one of the electrons "spin up" and the other "spin down"

Scandium : atomic number (Z) = 21 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the s and p orbitals of the third energy level (M shell), and, one pair of electrons occupy the s orbital of the fourth energy level (N shell)
Scandium is the first of the "d block" elements, so we now need to include a set of five d orbitals in our orbital diagram, with one electron occupying one of these boxes.

electron configuration (shells): 2,8,9,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹ 4s²

condensed electron configuration: [Ar] 3d¹ 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and 4s orbitals, with one electron occupying a 3d orbital

Titanium : atomic number (Z) = 22 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and, one pair of electrons occupy the s orbital of the fourth energy level (N shell)
The electronic configuration of titanium also includes 2 electrons in 3d orbitals

electron configuration (shells): 2,8,10,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d² 4s²

condensed electron configuration: [Ar] 3d² 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and 4s orbitals, with 2 electrons occupying 2 of the 3d orbitals, so we apply Hund's Rule to maximise the number of unpaired electrons and give them parallel spin. This means that 2 of the 3d orbitals will be occupied by 1 electron and these two arrows will point in the same direction.

Vanadium : atomic number (Z) = 23 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the s and p orbitals of the third energy level (M shell), and, one pair of electrons occupy the s orbital of the fourth energy level (N shell)
The electronic configuration of vanadium also includes 3 electrons in 3d orbitals

electron configuration (shells): 2,8,11,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d³ 4s²

condensed electron configuration: [Ar] 3d³ 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and 4s orbitals, with 3 electrons occupying 3 of the 3d orbitals, so we apply Hund's Rule to maximise the number of unpaired electrons and give them parallel spin. This means that 3 of the 3d orbitals will be occupied by 1 electron and all these arrows will point in the same direction

Chromium : atomic number (Z) = 24 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the s and p orbitals of the third energy level (M shell).
Note that we need to place 6 electrons into 6 orbitals of very similar energy (4s, 3d_xy, 3d_xz, 3d_yz, 3d_x²−y² and 3d_z²), so the atom will gain greater stability by having all these orbitals half-filled, meaning that each of these orbitals will be occupied by just 1 electron.

electron configuration (shells): 2,8,13,1

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

condensed electron configuration: [Ar] 3d⁵ 4s¹

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, but in accordance with Hund's Rule only 1 electron occupies the 4s orbital, and all the 3d orbitals and all these electrons have parallel spin (arrows all point in the same direction)

Manganese : atomic number (Z) = 25 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy s and p orbitals of the third energy level (M shell).
Note that we need to place 7 electrons into 6 orbitals of very similar energy (4s, 3d_xy, 3d_xz, 3d_yz, 3d_x²−y² and 3d_z²), the atom will gain greater stability by pairing up electrons in the single 4s orbital rather than in one of the 5 available 3d orbitals.
We apply the Pauli Exclusion Principle to this pair of electrons, giving one of the electrons "spin up" and the other "spin down".

electron configuration (shells): 2,8,13,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s²

condensed electron configuration: [Ar] 3d⁵ 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z and 4s orbital, and only 1 electron occupies each of the 3d orbitals and these electrons have parallel spin (arrows pointing in the same direction) in accordance with Hund's Rule.

Iron : atomic number (Z) = 26 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the s and p orbitals of the third energy level (M shell).
Note that we need to place 8 electrons into 6 orbitals of very similar energy (4s, 3d_xy, 3d_xz, 3d_yz, 3d_x²−y² and 3d_z²), the atom will gain greater stability by pairing up electrons in the single 4s orbital and in one of the 5 available 3d orbitals.
We apply the Pauli Exclusion Principle to these pairs of electrons, giving one of the electrons "spin up" and the other "spin down".

electron configuration (shells): 2,8,14,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²

condensed electron configuration: [Ar] 3d⁶ 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, 4s orbital and one of the 3d orbitals, with only 1 electron occupying each of the other 3d orbitals and these electrons have parallel spin (arrows pointing in the same direction) in accordance with Hund's Rule.

Cobalt : atomic number (Z) = 27 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the s and p orbitals of the third energy level (M shell).
Note that we need to place 9 electrons into 6 orbitals of very similar energy (4s, 3d_xy, 3d_xz, 3d_yz, 3d_x²−y² and 3d_z²), the atom will gain greater stability by pairing up electrons in the single 4s orbital and in two of the 5 available 3d orbitals.
We apply the Pauli Exclusion Principle to these pairs of electrons, giving one of the electrons "spin up" and the other "spin down".

electron configuration (shells): 2,8,15,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s²

condensed electron configuration: [Ar] 3d⁷ 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, 4s orbital and two of the 3d orbitals, with only 1 electron occupying each of the other 3d orbitals and these electrons have parallel spin (arrows pointing in the same direction) in accordance with Hund's Rule.

Nickel : atomic number (Z) = 28 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the s and p orbitals of the third energy level (M shell).
Note that we need to place 10 electrons into 6 orbitals of very similar energy (4s, 3d_xy, 3d_xz, 3d_yz, 3d_x²−y² and 3d_z²), the atom will gain greater stability by pairing up electrons in the single 4s orbital and in three of the 5 available 3d orbitals.
We apply the Pauli Exclusion Principle to these pairs of electrons, giving one of the electrons "spin up" and the other "spin down".

electron configuration (shells): 2,8,16,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s²

condensed electron configuration: [Ar] 3d⁸ 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, 4s orbital and three of the 3d orbitals, with only 1 electron occupying each of the other 3d orbitals and these electrons have parallel spin (arrows pointing in the same direction) in accordance with Hund's Rule.

Copper : atomic number (Z) = 29 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the s and p orbitals of the third energy level (M shell).
Note that we need to place 11 electrons into 6 orbitals of very similar energy (4s, 3d_xy, 3d_xz, 3d_yz, 3d_x²−y² and 3d_z²), the atom will gain greater stability in accordance with Hund's Rule by pairing up electrons in the five 3d orbitals and leaving just 1 electron in the 4s orbital
We apply the Pauli Exclusion Principle to these pairs of electrons, giving one of the electrons "spin up" and the other "spin down".

electron configuration (shells): 2,8,18,1

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹

condensed electron configuration: [Ar] 3d¹⁰ 4s¹

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and each of the five 3d orbitals, with only 1 electron occupying the 4s orbital

Zinc : atomic number (Z) = 30 (d block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), and 8 electrons occupy the s and p orbitals of the third energy level (M shell).
Note that we will place 12 electrons into 6 orbitals of very similar energy (4s, 3d_xy, 3d_xz, 3d_yz, 3d_x²−y² and 3d_z²), so that there will be a pair of electrons in each of these orbitals.
We apply the Pauli Exclusion Principle to these pairs of electrons, giving one of the electrons "spin up" and the other "spin down".

electron configuration (shells): 2,8,18,2

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s²

condensed electron configuration: [Ar] 3d¹⁰ 4s²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and each of the five 3d orbitals, and the 4s orbital

Gallium : atomic number (Z) = 31 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), 18 electrons occupy the completed third energy level (M shell), and, 2 electrons occupy the s orbital of the fourth energy level (N shell).
Now we add an electron to one of the p orbitals of the fourth energy level (remember there are 3 p orbitals making up the p-subshell)

electron configuration (shells): 2,8,18,3

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹

condensed electron configuration: [Ar] 3d¹⁰ 4s² 4p¹

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and each of the five 3d orbitals, and the 4s orbital. One electron occupies a 4p orbital

Germanium : atomic number (Z) = 32 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), 18 electrons occupy the completed third energy level (M shell), and, 2 electrons occupy the s orbital of the fourth energy level (N shell).
Now we add two electrons to the p orbitals of the fourth energy level (remember there are 3 p orbitals making up the p-subshell)

electron configuration (shells): 2,8,18,4

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p²

condensed electron configuration: [Ar] 3d¹⁰ 4s² 4p²

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and each of the five 3d orbitals, and the 4s orbital. Two electrons occupy 4p orbitals singly
We apply Hund's Rule to maximise the number of unpaired electrons, so the 2 electrons will occupy different 4p orbitals and will have parallel spin (arrows pointing in the same direction)

Arsenic : atomic number (Z) = 33 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), 18 electrons occupy the completed third energy level (M shell), and, 2 electrons occupy the s orbital of the fourth energy level (N shell).
Now we add three electrons to the p orbitals of the fourth energy level (remember there are 3 p orbitals making up the p-subshell)

electron configuration (shells): 2,8,18,5

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³

condensed electron configuration: [Ar] 3d¹⁰ 4s² 4p³

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and each of the five 3d orbitals, and the 4s orbital. Three electrons occupy 4p orbitals singly
We apply Hund's Rule to maximise the number of unpaired electrons, so the 3 electrons will occupy different 4p orbitals and they will have parallel spin (arrows pointing in the same direction)

Selenium : atomic number (Z) = 34 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), 18 electrons occupy the completed third energy level (M shell), and, 2 electrons occupy the s orbital of the fourth energy level (N shell).
Now we add four electrons to the p orbitals of the fourth energy level (remember there are 3 p orbitals making up the p-subshell)

electron configuration (shells): 2,8,18,6

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴

condensed electron configuration: [Ar] 3d¹⁰ 4s² 4p⁴

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and each of the five 3d orbitals, and the 4s orbital. Four electrons occupy the three 4p orbitals
We apply Hund's Rule to maximise the number of unpaired electrons and give these parallel spin (arrows pointing in the same direction), this means that one of the 4p orbitals must be occupied by a pair of electrons.
Apply the Pauli Exclusion Principle so that one electron of the pair is defined as "spin up" and the other as "spin down".

Bromine : atomic number (Z) = 35 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), 18 electrons occupy the completed third energy level (M shell), and, 2 electrons occupy the s orbital of the fourth energy level (N shell).
Now we add five electrons to the p orbitals of the fourth energy level (remember there are 3 p orbitals making up the p-subshell)

electron configuration (shells): 2,8,18,7

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁵

condensed electron configuration: [Ar] 3d¹⁰ 4s² 4p⁵

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and each of the five 3d orbitals, and the 4s orbital. Five electrons occupy the three 4p orbitals
Apply the Pauli Exclusion Principle so that one electron of the pair is defined as "spin up" and the other as "spin down".

Krypton : atomic number (Z) = 36 (p block element)
Aufbau Principle: 2 electrons occupy the completed first energy level (K shell), 8 electrons occupy the completed second energy level (L shell), 18 electrons occupy the completed third energy level (M shell), and, 2 electrons occupy the s orbital of the fourth energy level (N shell).
Now we add six electrons to the p orbitals of the fourth energy level (remember there are 3 p orbitals making up the p-subshell)
Each of the three 4p orbitals must be occupied by a pair of electrons.

electron configuration (shells): 2,8,18,8

electron configuration (sub-shells): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶

condensed electron configuration: [Ar] 3d¹⁰ 4s² 4p⁶

orbital diagram (orbital box diagram) : Pairs of electrons occupy the 1s, 2s, 2p_x, 2p_y, 2p_z, 3s, 3p_x, 3p_y, 3p_z, and each of the five 3d orbitals, the 4s orbital, and the three 4p orbitals
Apply the Pauli Exclusion Principle so that one electron of the pair is defined as "spin up" and the other as "spin down".

Worked Example of an Orbital Diagram Problem

Question 1: The ground state electronic configuration for an atom of oxygen is 1s² 2s² 2p⁴.
Draw the orbital diagram to represent O^2-.

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

   Draw the orbital diagram for O^2-

2. What data (information) have you been given in the question?

   Extract the data from the question:

   electronic configuration of O : 1s² 2s² 2p⁴

3. What is the relationship between what you know and what you need to find out?

   (a) Draw orbital box diagram for O: 1s² 2s² 2p⁴

   (i) Apply Hund's Rule :

   Maximise unpaired electrons in the orbitals of a subshell before pairing up electrons

   Unpaired electrons have parallel spin

   (ii) Apply Pauli Exclusion Principle:

   2 electrons in the same orbital have antiparallel spin

   (b) O^2- will have 2 more electrons than an O atom

   (i) Apply Aufbau Principle to add 2 more electrons to orbital diagram for O:

   1s

   →

   2s

   →

   2px

   2py

   2pz

   →

   3s

   (ii) Apply Hund's Rule :

   Maximise unpaired electrons in the orbitals of a subshell before pairing up electrons

   Unpaired electrons have parallel spin

   (iii) Apply Pauli Exclusion Principle:

   2 electrons in the same orbital have antiparallel spin

4. Work through the steps to draw the orbital diagram for O then o^2-

   (a) Draw orbital box diagram for O: 1s² 2s² 2p⁴

   (i) Apply Hund's Rule :

   Maximise unpaired electrons in the orbitals of a subshell before pairing up electrons

   4 electrons in 2p subshell which is composed of 3 p orbitals,

   A pair of electrons in 1 of the p orbitals, the other 2 p orbitals occupied by single electrons

   Unpaired electrons have parallel spin

   2 of the p orbitals occupied by single electrons with parallel spin

   (ii) Apply Pauli Exclusion Principle:

   2 electrons in the same orbital have antiparallel spin

   Pairs of electrons in 1s, 2s, and one of the 2p orbitals have antiparallel spin.

   orbital diagram for O	↑↓		↑↓		↑↓	↑	↑
   	1s		2s		2px	2py	2pz

   (b) O^2- will have 2 more electrons than an O atom

   (i) Apply Aufbau Principle to add 2 more electrons to orbital diagram for O:

   1s

   →

   2s

   →

   2px

   2py

   2pz

   →

   3s

   2 electrons will be added to 2p subshell

   (ii) Apply Hund's Rule :

   Maximise unpaired electrons in the orbitals of a subshell before pairing up electrons

   All electrons will be paired

   (iii) Apply Pauli Exclusion Principle:

   2 electrons in the same orbital have antiparallel spin

   Pairs of electrons occupy the 1s, 2s and each of the 2p orbitals

   orbital diagram for O2-	↑↓		↑↓		↑↓	↑↓	↑↓
   	1s		2s		2px	2py	2pz

5. Is your answer plausible?

   An atom of oxygen has 8 electrons.
   O^2- has 8 + 2 = 10 electrons.
   O^2- is isoelectronic with an atom of neon
   Electronic configuration of Ne (shells): 2,8
   Electronic configuration of Ne (subshells): 1s² 2s² 2p⁶
   Electronic configuration of Ne (orbital notation): 1s² 2s² 2p_x² 2p_y² 2p_z²
   Orbital diagram for an atom of Ne in ground state:

   orbital diagram for Ne	↑↓		↑↓		↑↓	↑↓	↑↓
   	1s		2s		2px	2py	2pz

   Since the orbital diagram for an atom of Ne agrees with the orbital diagram for O^2-, we are confident that our answer is plausible.

6. State your solution to the problem "Orbital diagram for O^2-":

   ↑↓		↑↓		↑↓	↑↓	↑↓
   1s		2s		2px	2py	2pz